1. Field of the Invention
The present disclosure generally relates to gamma correction and, more particularly, to a method of creating a gamma correction table for forming an image on a recording medium based on digital image data or an image processing method using a gamma correction table.
2. Description of the Related Art
In image forming devices for forming images on a recording medium based on a digital image data, each of the image forming devices has a different output density characteristic. Therefore, when forming an image on a recording medium based on the same digital image data, the density output on the recording medium may be sometimes different. In order to output a desired digital image data on a recording medium, each of the image forming devices generally corrects the digital image data according to the characteristic of each image forming device. This is well-known as gamma correction.
Also, in many cases, the tone number available per pixel in an image forming device such as a printer is smaller than the tone number of a digital image data handled in a personal computer or the like. Therefore, the digital image data is subjected to low tone processing to convert the tone number of the digital image data into a tone number which the image forming device can output. Among low tone processing, the screen processing generally converts input digital image data into image data that represents the tone in a quasi-manner in a certain area.
On the other hand, when performing the screen processing on the image data, the resolution may sometimes degrade. In particular, it is known that, in an image drawn with characters and/or lines, the characters and/or lines may generate a discontinuation, resulting in a considerable detone of the image quality. Japanese Patent Application Laid-Open No. 2005-341249 discusses a method in which a contour portion of an image is detected and the detected image is subjected to an enhancing or reducing processing, and subsequently the image is subjected to a continuous tone processing before being output. Pixels which are not in the contour portion are output after being subjected to the screen processing. According to the Japanese Patent Application Laid-Open No. 2005-341249, a contour portion and an area other than the contour portion are subjected to a different gamma correction.
A gamma correction table used for gamma correction is generated based on the characteristic of the output density, which is obtained by measurement of the actual density recorded on a recording medium, with use of a sensor.
However, conventional methods have a problem that it is difficult to obtain a precise characteristic of the output density. In the method discussed in the Japanese Patent Application Laid-Open No. 2005-341249, the tone on a contour portion is represented by a single pixel. On the other hand, in a portion other than the contour portion, an area including a plurality of pixels represents the tone in a quasi-manner. Therefore, a plurality of gamma correction tables appropriate to the respective areas has to be generated. However, in the conventional methods, appropriate gamma correction table is hardly generated.
According to the Japanese Patent Application Laid-Open No. 2005-341249, since dot gain and bleeding of ink have to be considered, the output density in a contour portion corresponding to a pixel of interest is measured using a sensor having the resolution lower than the resolution of input image data. However, it is likely that different output densities corresponding to the respective input tone values appear in a mixed state around the pixel of interest forming the contour portion. Therefore, since it is impossible to relate the output densities measured by a sensor to the input tone values in a one-to-one relationship, it is impossible to obtain an appropriate characteristic of the output density. As a result, it is difficult to generate a gamma correction table. Thus, it is occasionally necessary to correct the measurement of the output density to calculate the characteristic of the output density based on a measurement result of the output density recorded on a recording medium measured by a sensor of an image forming device.